France is expanding the definition of what kinds of clothes are unacceptable under the rules of laïcité, the country's strict separation of religion and state. For nearly two decades, public schools have barred students from wearing a visible Christian cross, a Jewish kippah, a Muslim headscarf or any other religious symbol deemed ostentatious by school officials.

But the abaya--a long, cloak-like covering--was a gray area until recently. The garment doesn't cover the head or face, but Muslim women in parts of North Africa and the Middle East traditionally wear it with a headscarf. In France, female students began wearing the abaya--without a headscarf in the classroom--as an extra layer of clothing because it covers their arms and legs, in compliance with what they say are Muslim teachings.

With the new school year about to start, President Emmanuel Macron's education minister stepped in and banned the abaya, ratcheting up France's long-running culture war over how far the government should go in enforcing laïcité rules in a country that is home to one of Europe's biggest Muslim minorities--estimated to be around 9% of the French population. Muslim leaders said the very definition of what constitutes an abaya is vague, opening the door to discrimination against Muslim students.